Looking at it another way: someone has a 1000sf house, someone else has a 4000sf house.

The guy w the 1000sf house wants to be just as warm as the guy w the 4000sf home, so he puts in the same size furnace, and sets it to run just as long as the one in the 4000sf house... then he can't understand why his friend's house is a comfortable 70F, while his house is pushing 120F

Say you buy a bag of balloons which can be blown up to eight feet tall. You blow up one to a height of two feet and draw a picture on the balloon using a stencil and some magic markers. I blow up another balloon to a height of four feet and stencil on the same picture, but it's proportionately larger to fill up the same percentage of the surface area as on your two-foot balloon. The density of marker ink is the same on your balloon and mine, but I use four times as much ink, right?

Now somebody comes along and thinks the balloons are beautiful, and they want to blow them up to eight feet tall and display them. Which balloon will now have the higher density of ink per unit of surface area, yours or mine? Which balloon is likely to look better?

This is what people keep missing. Yes, f/2 is f/2, and it puts the same density of light on the sensor. For the ten thousandth time, nobody denies that. However, you have to blow up the image from the smaller sensor a lot more than you do the image from the larger sensor in order to get the same display size. What matters is the density of protons used to form the final image, not the density used to form the image on the sensor.

Edit: All this is not to say that the smaller sensor is inferior, and anybody who thinks this is the natural conclusion is missing the whole point of the discussion. I don't have time to explain why, but a little thinking ought to reveal the answer.